The present invention relates to the surface treatment of materials such as metallic alloys and composites that have a tendency to oxidise in air. Examples of such materials include Pe, Ni, Cr, Co, Ti, Li, Mg, Zn, Al based alloys and super alloys. Other example materials include metal-matrix composites, single crystals and directionally solidified alloys or super alloys. Such metallic alloys and composites oxidise readily because of the high chemical affinity of the metal for oxygen. This property has, for many years, been employed to particular advantage in that the almost instantaneous formation of oxide layers on the surfaces of such materials forms an excellent barrier to further oxidation. However, the almost instantaneous formation of the oxide layer can be a significant obstacle when wishing to join or coat such materials. For example, it is a major obstacle during diffusion bonding, brazing and electroplating, as well as when manufacturing electrical or electronic components, heat sinks, etc in which it is necessary to make good electrical and thermal contact with the substrate but such contact is hindered by the existence of the oxide layer.
Many different approaches to oxide layer removal have been suggested as removal of the oxide layer is a necessary step in many processes, such as the brazing of aluminium alloys. One approach to removal is to heat the aluminium-based material to a brazing temperature in the presence of a flux in order to remove the oxide layer. The flux is usually toxic and corrosive, and any excess may be removed after brazing, and typically is chloride or fluoride-based for aluminium alloys. Chemical treatment with an acid or alkaline simply results in the replacement of the aluminium oxide layer with various types of sulphides, nitrides, hydroxides, etc., rather than producing the desired oxide-free surface.
Another known method for the removal of the oxide surface is the use of ion beam cleaning in a vacuum, which must then be followed by in-situ sputter coating of another metal (such as copper or silver) on the clean surface to prevent re-oxidation when the surface is exposed to air. As will be appreciated, this approach is expensive, requires complex equipment and procedures, and therefore is of restricted use.
There have also been proposals to employ gallium in combination with aluminium for bonding or brazing soldering. For example, EP-A-0123382 proposes several different methods for bonding aluminium using gallium by rubbing molten gallium or an aluminium-gallium alloy directly on to the surface of aluminium or by employing chemical deposition by dipping aluminium in a solution of nitric acid containing gallium nitrate followed by lengthy heat treatment. There is also discussion in this document of electrochemical disposition using a gallium nitrate electrolyte. However, as has been known for a number of years, aluminium is attacked by molten gallium which then embrittles the aluminium to an extent that it can be damaged even by simple touch with a fingernail. So, this prior art document indicates that it is necessary to employ long bonding and/or heat treatment times (for example seventy to eighty hours) making the bonding a lengthy and impractical process.
In relation to brazing soldering, U.S. Pat. No. 2,824,365, for example, proposes the rubbing of gallium onto an aluminium surface in order to improve joint properties prior to soldering the aluminium by use of a lead-tin alloy. The detrimental effect of rubbing gallium is referred to in this prior art document to the extent that it is recommended to wipe off the crumbled surface of aluminium prior to soldering.
The present invention seeks to provide a method of oxide layer removal that overcomes the above problems.
According to the present invention there is provided a method for treating the surface of readily oxidisable material to remove an oxide layer formed thereon, the method comprising the step of:
grinding the surface of the material with a grinding or polishing device having a metal with a melting point of 300xc2x0 C. or lower impregnated therein.
The readily oxidisable material may be one of iron, nickel, lithium chromium, cobalt, titanium, copper, magnesium, aluminium or zinc based alloy or super alloy, metal matrix composite, single crystal or directionally solidified alloy or super alloy.
The grinding or polishing device may be a rotating or oscillating grinding tool, may be a grinding brush, or may be a cloth or paper. The metal may be gallium, indium zinc, or mercury or their alloys. The method may further comprise the step of heating the surface of the base material as it is ground, although this is not always needed.
The present invention provides an end component which is protected from further oxidation by the impregnating metal layer that is formed through the grinding process, but as only a very thin layer is produced there are none of the problems associated with embrittling that occurs in the prior art referred to above.